The moderate guild
A moderate guild will see the least impact from the nerf, but it'll still be felt in a positive way. Chances are, if you're in a moderate guild (like I am), then the normal modes are cleared and hard modes are being worked on, albeit at a potentially slower pace than might be ideal.
The moderate guild will see some of the roadblocks standing in their hard mode raiding way evaporate. (Think if heroic Morchock has just a bit fewer HP -- no more 2% wipes -- or if Hagara's lightning did 5% less damage). These guilds will also have an easier and more relaxed time farming content to help them get their overall gear levels up. Yes, Spine and Madness are on farm, but there's still an enrage timer on Madness that has to be met, and an unfortunate mistake or two can erase 15 minutes of hard work. Those mistakes? Less likely to be deadly now, and that's a good thing.
Finally, the moderate raiding guild will have an easier time gearing people up to fill their ranks. Chances are the moderate guild doesn't have heavy standards when it comes to gearing but still requires at least a 384 level or so. With this nerf, maybe 378 would be OK if the person is well experienced or shows initiative and has a positive entrance interview. After all, if the boss has 10% less health, that means 10% less DPS needs to be done. Gearing happens more easily, and the people available to be geared up increases.
The hardcore guild
In my opinion, the hardcore guild has the most to gain from Dragon Soul nerfs.
But let's get something out of the way first -- the hardcore guilds can turn this buff off. All they need to do is talk to an NPC at the beginning of the instance, and the nerf goes away. They'll then be attempting the encounters as close to their original release as possible (sans specific changes in hotfixes/patches, of course).
Now, as with the moderate guilds, they're going to see an easier time getting people geared up and contributing to the raid. The hardest part of running a hardcore guild isn't the encounters; it's finding the dedicated people to raid at the consistent high level you demand. This buff? A player will be (in total) 10% more likely to contribute at the necessary level. If there are 100 players in a room and only 10 of them can raid at a hardcore level, hardcore guilds now have an 11th person they can pick. That means it's easier to deal with people's real-life emergencies and easier to deal with the inevitable burnout that comes from hardcore raiding at the end of an expansion cycle.
And there is perhaps the most important aspect of this nerf, more so than anything else and the one aspect that makes this nerf the most attractive to these types of guilds: It allows them to continue to progress through the hard modes at a pace that keeps them alive, even though many members want to take time off. If these guilds are the base of WoW that keeps our game alive, as some would contend, then Blizzard is directly giving them the tools they need to survive.
I never understand when hardcore guilds complain that the encounters themselves become easier. Mechanically, they're just as hard. Positioning, coordination, overall gameplay and communication -- all those things are just as difficult before and after the nerf. The only thing that has always been easy is gearing and DPS. Anyone can get gear and do passable DPS; not everyone can follow directions or possesses the coordination and skills at the level necessary for heroic encounters. This nerf will allow hardcore guilds to seek out those good players and wrap them up in their community, something I know many guild leaders desperately want to do.
Back in the day when I ran a hardcore guild, I would gladly gear a fresh max-level character if I knew beyond a shadow of a doubt that the player performed at a professional level. This nerf? This nerf allows that mindset to begin to happen again.
Even those that disagree ...
Of course, not everyone agrees that this is a good thing, and that's A-OK. Disagreement is the beauty of the system. Because there's an easy answer if everything I said in this editorial rings false to you.
Just turn the nerf off.